Your sister should contact the FBI, as it is unlikely that she is the scammers' only target, and this crime likely crosses state (or international) borders.
No, the FBI is not precluded from investigating cross-border crimes. They just have to work with their foreign counterparts on the investigation.
And the amounts cited are not below the FBI's interest threshold; the whole point of my comment is that their are probably other victims, which makes this a larger crime than just the OP's system and probably involves sophisticated or organized criminals. This makes it exactly the kind of crime the FBI loves to investigate.
Only 4 countries (Micronesia, North Korea, Palau and Tuvalu) are not participants in INTERPOL, and criminal organizations generally don't operate out of them...
Your sister should contact the FBI, as it is unlikely that she is the scammers' only target, and this crime likely crosses state (or international) borders.